A film video player for the television display of color images recorded on color negative photographic film has been publicly demonstrated. The film video player, described in pending U.S. application Ser. No. 426,426, filed Sept. 29, 1982 by L. G. Moore and T. H. Lee, and in corresponding PCT International Application U.S. 83/01416 filed Sept. 19, 1983, includes a large CCD image sensor of the type in which photosignals are generated directly in the transfer channels of the CCD. No light shielded frame storage array is provided on the image sensor so that the entire area of the image sensor can be employed to sense light, thus maximizing the resolution and light collecting capacity of the sensor. The image sensor is illuminated by a pulsed light source during the vertical retrace interval of a standard television signal, and then read out in the dark to avoid smearing the image signal generated by the image sensor. The pulsed light source in the film video player that was demonstrated to the public comprised a quartz halogen projection lamp, the light output from which was chopped by a spinning shutter disc. Although the light source produced a very adequate illumination with excellent pulse-to-pulse uniformity and constant color temperature, the chopped light source was mechanically complex and consumed a considerable amount of energy, requiring substantial cooling.
In the above mentioned patent application, it is noted that alternatively, the pulsed light source may comprise an electronic strobe light such as a Xenon flash lamp synchronized to the vertical retrace interval of the television signal. The film video player with an electronic strobe light would provide several advantages over a film video player with a quartz halogen projection lamp with a mechanical shutter. Namely, the electronic strobe lamp would provide reduced mechanical complexity and lower power consumption, resulting in reduced heat generation and consequently, reduced cooling requirements.
It has been found however, that a film video player of the type described above having a conventional electronic strobe light as the pulsed light source, produces a television signal that evidences an annoying flicker. A number of flash tubes were tested in a conventional stobe light circuit and it was found that the average flash-to-flash variation in light output was on the order of one stop (0.3 log E). The most stable of the flash tubes tested produced a variation of approximately 0.1 stop of exposure variation flash-to-flash. The use of these flash tubes in the film video player resulted in a very visible flicker on the television display. A conventional flash quench circuit with a closed loop servo for measuring the light output and quenching the flash when the output reached a determined value was found to reduce the measured flicker to approximately 0.02 to 0.05 stops of exposure from flash-to-flash. This level of flash-to-flash variation was still noticeable on a television display of still pictures. When the amount of light output is changed with this type of flash circuit, the color temperature of the light shifts, causing an unwanted color change in the video display.